Steric stabilization, polymeric surfactant adsorption

Surfactants have also been used to overcome the solubility limitation of synthetic polymers in CO2 (most common synthetic polymers would be considered to be C02-phobic). For example, surfactants have been used to aid in the dispersion polymerization of poly(methylmethacrylate) (PMMA) in CO2 (58-60). The surfactants used in the polymerizations of PMMA are more accurately referred to as stabilizers. The C02-phobic region acts as anchor to the growing polymer, either by physical adsorption or by chemical grafting. The C02-philic region sterically stabilizes the growing polymer particles, preventing flocculation and precipitation. When a biopolymer is not soluble in CO2, specific surfactants may be designed to aid in the solubilization of the polymer into CO2. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

From the above discussion, one can summarize the most important criteria for effective steric stabilization when using polymeric surfactants. First, there should be enough polymer to ensure complete coverage of the particle surface by the chains. This will prevent any attraction between the bare patches of the particles, while it also prevents any bridging flocculation (simultaneous adsorption of the chain on more than one particle). 

In Chapter 3, the solution and surface properties of a relatively new class of material, namely, polymeric surfactants, are illustrated in some detail using Flory-Huggins theory and current polymer-adsorption theory. This is followed by a discussion of the phenomenon of steric stabilization of suspended particles and how it is affected by the detailed structure of the stabilizing polymeric species. It concludes with a discussion of the stabilization of emulsions by interfacial and bulk theological effects, and presents closing comments on multiple emulsions. 

As will be discussed later, one of the main features of effective steric stabilization is strong adsorption (anchoring) of the chains to the interface. This anchoring is produced by the use of polymeric surfactants, the main topic of this chapter. These materials have attracted considerable attention in recent years for stabilization of many o/w and w/o emulsions. Apart from their effectiveness in prevention of flocculation and coalescence of the droplets, they are also expected to cause no skin irritation. The high molecular weight of the surfactants prevents their penetration through the skin and hence they do not cause any disruption of the stratum corneum. 

It can be seen from the above short discussion that agrochemical formulations are complex multiphase systems and their preparation, stabilization and subsequent application require the application of the basic principles of colloid and interface science and this is the objective of the present part [1, 2]. It will start with a section on surfactants and the physical properties of their solutions. This is followed by a section on the interfacial aspects of agrochemical formulations including adsorption of surfactants and polymeric surfactants at the air/liquid, liquid/liquid and solid/liquid interfaces. The stabilization of dispersions, both electrostatic and steric, is discussed in the next section. The basic principles of colloid and interface science are illustrated in detail by considering emulsion concentrates (EW s) and suspension concentrates (SC s). A summary will be given on microemulsions and controlied release of agrochemical formulations. 

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